The Messenger of Allah ﷺ wrote to Caesar inviting him to Islam and sent his letter with Dihya Kalbi. The Messenger of Allah ﷺ instructed him to deliver it to the governor of Busra, who would in turn pass it on to Caesar.When Allah had repelled the Persian armies from Caesar, he walked from Homs to Ilya in gratitude for what Allah had granted him. When the letter of the Messenger of Allah ﷺ reached Caesar, he read it and said, "Find me someone here from his people so I may ask them about the Messenger of Allah ﷺ."
أن رسول الله ﷺ كتب إلى قيصر يدعوه إلى الإسلام، وبعث بكتابه إليه مع دحية الكلبي، وأمره رسول الله ﷺ أن يدفعه إلى عظيم بصرى ليدفعه إلى قيصر، وكان قيصر لما كشف الله عنه جنود فارس مشى من حمص إلى إيلياء شكرا لما أبلاه الله، فلما جاء قيصر كتاب رسول الله ﷺ قال حين قرأه: التمسوا لي ها هنا أحدا من قومه لأسألهم عن رسول الله ﷺ.
Bukhari narrated it in the Book of Jihad and Expeditions (2940) from Ibrahim bin Hamza, who said, Ibrahim bin Saad narrated to us, from Salih bin Kaysan, from Ibn Shihab, from Ubaydullah bin Abdullah bin Utba, from Abdullah bin Abbas, who said, and then he mentioned it.